The present invention relates to an apparatus for isolating a radar receiver from a radar transmitter in CW (continuous wave) radar applications utilizing a single antenna for transmitting and receiving.
In some transmitter/receiver applications such as radar, it is desirable to utilize the same antenna for both transmitting and receiving, thus saving the space required for a second antenna and maximizing the antenna gain. It may also be desired to transmit and receive on essentially the same frequency simultaneously as in the manner of CW Doppler radar or a type of frequency diverse radar where the diversity is a relatively small fractional bandwidth. In all cases but the simplest and lowest power, there is a need for a means to reduce the amount of transmitter signal that reaches the receiver input to reduce received transmitter noise and prevent receiver overload. This class of transmitter/receiver has transmitter and receiver frequencies so close to each other that satisfactory isolation of the receiver from the transmitter power by frequency filters alone is not possible.
In some low power applications, a non-reciprocal, multi-port isolation device such as a circulator is used to interconnect the transmitter, antenna and receiver. Such a device has the unique property that power flow between any two of its ports proceeds at low loss in the forward direction but has high loss in the reverse direction. The high loss direction can reduce the amount of transmitter power reaching the receiver input by approximately 25 to 30 db. This improvement in transmitter to receiver isolation allows higher transmitter power and therefore increased range.
As transmitter power is increased, a point is reached where receiver overload again occurs, the limit being set by the residual leakage signal of the circulator and by the reflection of the transmitter signal at the antenna. Careful tuning of the antenna and/or circulator can make further improvements in isolation, but the sensitivity of adjustment to environmental factors is increased and the isolation bandwidth is decreased.
The leakage signal that is transmitted by the circulator from the radar transmitter to the radar receiver poses two threats to the receiver. As the transmitter power increases, the leakage signal level increases and approaches the level of magnitude that will cause burnout of semiconductor devices in the input section of the receiver. Further, as the level of the leakage signal increases, but before the burnout level is reached, the leakage signal contributes to the noise figure of the receiver and reduces the achievable signal to noise ratio of the radar system.
The need to reduce the leakage transmitted by a circulator from the transmitter to the receiver has led to the concept of reflection of a portion of the transmitter signal by the isolation apparatus of the present invention. The isolation apparatus of the invention is arranged to intercept a portion of a transmission signal supplied through the circulator to the antenna and to reflect that portion back to the circulator with magnitude and phase characteristics which will, when the reflected portion is combined in the circulator with the leakage portion, cancel the leakage signal. The cancellation is effected over a predetermined frequency range corresponding to the major portion of the leakage signal spectrum. Thus, the performance of the invention is distinguishable from prior art assemblies such as double stub tuners and EH tuners that are used to reflect narrow band components; as is known, such narrow band return effectively cancels unwanted leakage signal power only at the center of the leakage signal band.
As isolation is improved by the apparatus of the invention through the provision of leakage signal cancellation over a relatively substantial leakage signal frequency range, more transmitter power can be used with concomitant improvements in radar system sensitivity and range. Further, since transmitter noise is reduced by this technique, receiver sensitivity can remain high.
Unless the circuitry is carefully designed, the bandwidth of operation will be severely limited and the susceptibility to environmental factors increased. The vectors which are summed must have low phase and amplitude sensitivities with respect to each other for best overall performance. Since the several vectors arrive at the summing point by different physical and electrical path lengths, their relative phases tend to diverge as bandwidth is increased causing a non-zero vector sum and loss of isolation. Reducing the spacial distribution of the circuitry and matching of electrical path lengths are necessary conditions for improving operating bandwidth.